Soft magnetic powdered cores produced by die compacting of a powder of soft magnetic metal such as iron, have a superior material yield at the time of production as compared with laminate cores using an electrical steel sheet or the like, and the material cost can be thus reduced.
Furthermore, since soft magnetic powdered cores have a high degree of freedom in shape designing, it is possible to improve their characteristics through optimal shape designing of the core. It is also possible to reduce the eddy current loss thereof to a large extent, by mixing an electrically insulating material such as a resin powder into the metal powder to insert the insulating material between the particles of the metal powder and increase the electrical insulation between them. The cores thus obtained are possible to exhibit excellent properties particularly in a high frequency range.
On the other hand, due to the insulating material such as a resin inserted between the particles of a soft magnetic powder, soft magnetic powdered cores have a drawback that, if the amount of the insulating material making up the core is large, the amount per volume (space factor) of the soft magnetic powder decreases and the magnetic flux density also decreases. In order to address this drawback, Patent Document 1 as described below discloses a technique of reducing the amount of a resin powder added, by forming an inorganic insulating film on the surface of the soft magnetic powder and thereby enhancing the electrical insulation properties of the soft magnetic powder. In recent years, there is a demand for a further enhancement of magnetic properties and Patent Document 2 as described below suggests a soft magnetic powdered core having a further decreased amount of the resin powder added.
In order to enhance the magnetic properties of a soft magnetic powdered core, it is necessary to increase the space factor of the soft magnetic powder in the core. Accordingly, densification of soft magnetic powdered cores is desired and attempts have been made to perform compacting of the soft magnetic powder at a high pressure such as 1000 MPa or higher. However, if the soft magnetic powder is compressed at a high pressure, the residual compressive stress in the soft magnetic powdered core increases so that magnetic permeability and magnetic flux density are lowered and the hysteresis loss increases at the same time.
Thus, in order to improve the magnetic properties of the soft magnetic powdered core, attempts have been made to decrease the hysteresis loss by subjecting the soft magnetic powdered core to a heat treatment at a temperature lower than the sintering temperature so as to ease the strain caused by compression. Patent Document 3 discloses a method for producing a soft magnetic powdered core by compacting a powder mixture obtained by adding a small amount of an organic resin binder to a soft magnetic metal powder coated with an inorganic insulating film, and by heat-treating then the green compact thus obtained. As such, various methods have been proposed to achieve a good balance between high magnetic flux density and low iron loss in a soft magnetic powdered core.